Fix #25: Add a %d for quickfix title

Instead of %s (which isn't used), use %d to get the number items in the
quickfix/locationlist so you can see at a glance the magnitude of the
results.

Add %d where it was missing.

Add documentation.

You may not see results when using fancy statuslines like powerline. You
can test what it would display:
	:echo w:quickfix_title